Chem­i­cal thin­ning

The Orchardist - - Contents - By Jack Hughes

This ar­ti­cle is be­ing writ­ten as ap­ple bud move­ment in Hawke’s Bay is get­ting un­der­way. Pink is show­ing in blocks that have been treated with dor­mancy break­ers. It is be­fore the start of the chem­i­cal thin­ning sea­son now but by the time you read this, it will be done if not dusted. Strate­gies will be im­ple­mented and treat­ments ap­plied but the fi­nal out­come won’t yet be fully re­vealed. So that’s the con­text and time­line for this brief chat about chem­i­cal thin­ning – where to be­gin? A good place to start is con­sid­er­ing the ob­jec­tives with chem­i­cal thin­ning of ap­ples. Ta­ble 1 is an at­tempt to list them in some pri­or­ity or­der:

Ta­ble 1. Thin­ning ob­jec­tives

1. Good re­turn bloom – next year

2. A full crop – this year 3. Fruit re­tained on best sites 4. Uni­form crop load over the tree 5. A small and eco­nom­i­cal hand thin job Achiev­ing all five ob­jec­tives (or even scarier if they are called key per­for­mance in­di­ca­tors) is a tall or­der when ap­plied to ev­ery block on ev­ery or­chard. If we agree that points one and two are top pri­or­ity – uni­form re­turn bloom next year and suf­fi­cient fruit num­bers for a full crop this year, its sound­ing more doable. If points three and four are con­sid­ered as com­po­nents of a full crop, then maybe its sound­ing eas­ier still. Maybe it’s the fi­nal point, the fi­nan­cial mo­ti­va­tion for min­i­mal hand thin labour costs that is the cause for go­ing ‘too hard’ and over­do­ing it at the ex­pense of ob­jec­tives one to four? If fruit borne on the best spur sites have been ‘blown off’ by ‘hot brews’ and the eas­ily thinned bot­toms turn out shy on crop, we’ve failed at points three, four and prob­a­bly two!


The pre­vi­ous sea­son, 2016-7, was a cli­matic out­lier over the crit­i­cal stage of fruit­let thin­ning. Both Hawkes Bay and Nel­son ex­pe­ri­enced un­usu­ally low so­lar ra­di­a­tion and high night tem­per­a­tures for about seven days when the fruit size of many blocks was ‘in the zone’ (8-16mm fruit­let di­am­e­ter). Th­ese con­di­tions co­in­cided with a big in­crease in the use of a new chem­istry for ap­ple thin­ning. We’re talk­ing about the pho­to­syn­thetic in­hibitor, metamitron, which has a to­tally dif­fer­ent mode of ac­tion from the prod­ucts we have decades of ex­pe­ri­ence with.

The stan­dard bloom and fruit­let thin­ner prod­ucts are driven by tem­per­a­ture – the hot­ter it is over the days fol­low­ing ap­pli­ca­tion, the greater the ef­fi­cacy. It’s a re­sponse we un­der­stand and look for in plan­ning our tim­ing of ap­pli­ca­tion.

So, given a new prod­uct with a dif­fer­ent driver, namely car­bo­hy­drate deficit, a few ‘over­thin’ sur­prises were on the cards.


The Car­bo­hy­drate Model de­vel­oped at Cor­nell Univer­sity, New York, pro­vides grow­ers there with an on-line de­ci­sion sup­port tool to guide chem­i­cal thin­ning strat­egy and im­ple­men­ta­tion. The model uses cur­rent and fore­cast max­i­mum and min­i­mum tem­per­a­tures and so­lar ra­di­a­tion to pre­dict the tree’s sen­si­tiv­ity to nat­u­ral drop and re­sponses to chem­i­cal thin­ning.

More fruit­let drop and eas­ier thin­ning oc­curs fol­low­ing cloudy con­di­tions and high (es­pe­cially night) tem­per­a­tures. Trees are more sen­si­tive to thin­ning when their car­bo­hy­drate bal­ance is low­ered by re­duced pho­to­syn­thetic pro­duc­tion and higher res­pi­ra­tion de­mand (or cost) re­sult­ing from th­ese con­di­tions. This was the sit­u­a­tion in early Novem­ber, 2016 – see fig­ure 2 for the sit­u­a­tion in nor­mally sunny Nel­son.

We also learned how the ef­fects of ‘ar­ti­fi­cially in­duced shade’ i.e. the ap­pli­ca­tion of a pho­to­syn­thetic in­hibitor, could com­bine with nat­u­ral shade the cli­matic pre-con­di­tion­ing to give ex­ces­sive thin­ning in the less well-lit zones of trees. This has prompted a ‘re-think’ around the set-up and cal­i­bra­tion of spray­ing equip­ment.


Fruition put this rather tech­ni­cal sub­ject to the con­tes­tants at T&G’s an­nual Hor­tisports day in Hast­ings in Au­gust. Their chal­lenge, with no warn­ing and in front of a large crowd, was to ‘spray’ their tree with blue paint with a cov­er­age pat­tern that would re­sult in a uni­form crop over the whole tree. I think that the young con­tes­tants did an awe­some job in a high pres­sure sit­u­a­tion. They showed a great un­der­stand­ing of the fac­tors at play.Three sam­ples of their ‘cal­i­bra­tion art­work’ are shown in Fig­ure 3. Suc­cess­ful im­ple­men­ta­tion in the field presents the chal­lenge of turn­ing a tar­get into a re­sult.


It would be wrong to cre­ate the im­pres­sion that over-thin­ning was the only is­sue. Ap­ple va­ri­etal dif­fer­ences are pro­nounced and Fuji stands out as a chal­lenge be­ing both dif­fi­cult to thin and sus­cep­ti­ble to bi­en­nial bloom be­hav­ior.Add in its ten­dency to bloom over an ex­tended time which re­sults in a wide range of fruit­let sizes and you have a chal­leng­ing cus­tomer. While dor­mancy break­ers can help by com­pact­ing bloom, the new

“Will 2017 see large fruit sets and ex­pen­sive hand thin­ning jobs re­sult­ing from overly timid thin­ning pro­grammes? We should have a bet­ter idea by the time you read this.”

chem­istry strug­gled to pro­duce stun­ning re­sults even at higher rates and given the cli­matic con­text. There is still plenty more to learn here.


The in­tro­duc­tion of pow­er­ful fruit­let thin­ning chem­istry (metamitron) that has been shown to thin as much alone as when pre-ceeded with bloom thin­ners raises a ques­tion. Do we need to bother with bloom thin­ners any­more? Its fair to say that some­times bloom thin­ners don’t work, some­times they do harm and some­times they work well. A key fac­tor is how im­por­tant bloom thin­ners are in achiev­ing our first ob­jec­tive – good re­turn bloom.


Its great to have two groups of prod­ucts that re­spond to dif­fer­ent driv­ers – warm tem­per­a­tures for the plant growth reg­u­la­tors and car­bo­hy­drate deficits for the pho­to­syn­thetic in­hibitors. It does give the op­por­tu­nity to select the group best suited to the cli­matic con­di­tions. Not that I’m sug­gest­ing that th­ese de­ci­sions are a ‘walk in the park’.


The odds are that we will have ex­pe­ri­enced more nor­mal con­di­tions over bloom and fruit­set in 2017 than we did in 2016. That would mean more sun­shine and cooler nights re­sult­ing in more pos­i­tive car­bo­hy­drate bal­ances and lower sen­si­tiv­ity to fruit drop and chem­i­cal thin­ning. It would be un­der­stand­able if many grow­ers were in­clined to be cau­tious fol­low­ing un­wel­come sur­prises in 2016. Will 2017 see large fruit sets and ex­pen­sive hand thin­ning jobs re­sult­ing from overly timid thin­ning pro­grammes? We should have a bet­ter idea by the time you read this.

Th­ese im­ages demon­strate ideal cov­er­age pat­terns for chem­i­cal thin­ning.

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